The invention relates to a method of regenerating old casting or foundry sand by thermal and mechanical processing and, more particularly, to methods for regenerating old casting sand which make efficient use of thermal energy.
There are various molding sand systems currently used in foundries. Some use inorganic binders such as clay, and others use organic binders such as synthetic resin. Among the latter sand systems are those which sue binding agents containing a volatile solvent. Systems using organic binders are used primarily for the mask molding method while systems which use synthetic resin binders are used predominantly for core molding. Finished sand with clay as the binding agent is used primarily for the areas outside the core of a mold.
The expression "molding sand system with clay" is used to denote the cycle wherein finished sand is put into the mold, a molding operation is performed in the casting process, and the sand is discharged after the emptying operation in the form of old sand. The old sand is at least partly mixed again with fresh sand, bentonite, water and coal dust to make it useable again.
In such a molding sand system with clay, which is commonly used, there is a continuous need for fresh or new sand. That sand is required to compensate for losses and to replace damaged sand. Fresh sand is also used for producing cores which contain synthetic resin as the binder. Losses occur when emptying out the molds after the casting operation, such as when trimming, fettling, or cleaning out.
The quality of sand required for producing the cores is generally higher than that required for producing the other finished sand in the outer parts of the mold. Frequently, after a casting operation, cores are broken up to be recycled so that the high quality sand they contain is used to compensate for the lost amount of sand in the molding sand system with clay. In known molding sand systems wtih clay, the requirement for fresh sand is increasingly directed to the sand needed for the cores, and therefore, such sand must be fresh sand of high quality.
Experience has shown that in foundry plants, large quantities of old sand are dumped as waste after the molds have been emptied out, both to compensate for the surplus or excess due to the supply for the core, and to keep the average quality of the circulating old sand at a constant value. Much effort is currently underway to optimize processing of the waste sand in order to produce a new sand of such high quality that it can be used to form a core. Such efforts are partly motivated by economic considerations because the transportation and storage of used sand involves additional costs.
Although the current demand for lower quality sand can be filled by presently existing reprocessing plants, there is a need for new sand or processed old sand of a higher quality for making mold cores. In order to produce such higher quality sand, neither mechanical nor thermal regeneration of old sand alone is sufficient, but a combination of thermal and mechanical processing of old sand is required.
Such methods of regenerating old sand by thermal and mechanical processing are already known. For example, Offenlegungsschrift No. 31 03 030 describes a method in which old sand is introduced under quantitatively controlled conditions into a fluidized bed furnace and is heated and thermally processed by hot gases in such a way that the fine grain component can be separated off. In the furnace, all the old sand is dried such that the clay or bentonite loses its binding capability and its plasticity.
After the thermally treated sand mixture is cooled, the embrittled bentonite crusts around the quartz grains are fed to a striker-type crushing mill having a sifter disposed downstream thereof. In the mill, the solid binding agent residues or crusts are stripped off of the quartz grains. The fine component which is rubbed off is separated from the heavy quartz grains in a sifter.
Upon sufficiently intensive thermal and subsequent mechanical treatment of the old sand, that method can actually produce sand of a quality which approaches new sand. However, a disadvantage of this process is that it requires a relatively high amount of energy. Experiments have shown that this is becausae the entire volume of sand in the furnace must be heated to about 870.degree. C. (1598.degree. F.). On a microscope level, each quartz grain of the sand is heated to the final temperature to its core in order to ensure that all the binder components or bentonite casings around the quartz grains are at the requisite embrittling temperature during the mechanical processing operation. This occurs because the thermal conductivity of quartz is higher than that of the bentonite encasing it. Such heating involves the use of an undesirably large amount of heat.
Another type of sand regenerating system is shown in U.S. Pat. No. 2,456,769 to Christensen et al. In that system, spent foundry sand is dumped into a first crib where the sand is wetted, mixed and mulled. The wet scrubbed material is then conveyed to a second crib in metered quantities where it is simultaneously mulled, mixed and subjected to temperatures of between 900.degree. F. (480.degree. C.) and 1200.degree. F. (650.degree. C.). And finally, the cleaned, hot, dry sand is conveyed to a third crib where it is aerated, cooled and dry scrubbed.
A disadvantage of this process is that, in the second crib, wet sand is introduced to be heated, mixed and mulled. Consequently, much heat energy is lost in the production of steam and vapor which escapes from the crib. Furthermore, in the second crib the burners are positioned directly over the mixing and mulling apparatus, so that the mixing and mulling implements are heated directly by the burners. This direct heating of the implements at elevated temperatures disclosed in that patent would soften the implements and cause them to wear rapidly.
Therefore, there is a need for a process to regenerate old casting sand which requires less energy and a lower capital investment than prior art processes. Such a process preferably does not create excessive wear on the components of the apparatus which performs the process.